Many natural products which contain a chromophoric group capable of absorbing ultraviolet light undergo structural changes when exposed to sunlight. Such transformations are known either to bring about the formation of biologically active compounds or to bring about the destruction of such materials, or both. Of particular interest in this present research are natural materials which contain the dienic and trienic chromophoric grouping since these groupings are related to the formation and destruction of the antirachitic factor, vitamin D. The proposed research will continue to study the photochemistry related to vitamin D. The various structural parameters in the starting diene system will be evaluated, features such as ring size, stereochemistry, temperature, and wave length of absorption will be studied. Also the role of peripheral hydroxyl groups and the role of oxygen and/or nitrogen on or in the original chromophore will be evaluated. These studies should yield information related to the preparation of new substituted vitamin D analogs, materials which should be useful in detailing the exact mechanistic role of hydroxy-vitamin D. Since most of the photochemistry of materials related to natural products occur in protic solvents, the photoinduced reaction of polyenes with protic solvents will be studied. The study should yield information regarding the stability of vitamin D in the body. Also, cycloaddition between two unsaturated compounds is often catalyzed by ultraviolet light and the general aspects of this type of reaction will be studied.